Flexibility in animal signals facilitates adaptation to rapidly changing environments

Abstract

Charles Darwin posited that secondary sexual characteristics result from competition to attract mates. In male songbirds, specialized vocalizations represent secondary sexual characteristics of particular importance because females prefer songs at specific frequencies, amplitudes, and duration. For birds living in human-dominated landscapes, historic selection for song characteristics that convey fitness may compete with novel selective pressures from anthropogenic noise. Here we show that black-capped chickadees (Poecile atricapillus) use shorter, higher-frequency songs when traffic noise is high, and longer, lower-frequency songs when noise abates. We suggest that chickadees balance opposing selective pressures by use low-frequency songs to preserve vocal characteristics of dominance that repel competitors and attract females, and high frequency songs to increase song transmission when their environment is noisy. The remarkable vocal flexibility exhibited by chickadees may be one reason that they thrive in urban environments, and such flexibility may also support subsequent genetic adaptation to an increasingly urbanized world.

Figure 1. Relationship between bee note peak frequency and average ambient noise levels.

The line predicting bee note peak frequency is derived from a general linear mixed model and data points correspond to the observed mean bee note peak frequency at each decibel (±1 SE) averaged for all songs in all sites on weekend (black circles) and weekday (grey triangles) recordings.

Figure 2. Relationship between song duration and instantaneous noise.

The line predicting song duration is derived from a general linear mixed model and data points correspond to the observed mean song duration at each decibel (±1 SE) averaged for all songs in all sites on weekend (black circles) and weekday (grey triangles) recordings.